1. Field of the Invention
The present invention relates to a signal modulation method, a signal modulation apparatus, an electronic device, and a computer program product.
2. Description of the Related Art
Signal modulation apparatuses (sigma-delta modulation apparatus and delta-sigma modulation apparatus) that modulate an input signal, to carry out an analog-digital conversion and a digital-analog conversion have been widely applied to electronic devices such as audio equipment, communication equipment (for example, audio coder-decoder (CODEC) for mobile terminals), and measuring equipment that require highly accurate signal processing. Related technology has been disclosed in Japanese Patent Application Laid-open No. 2002-502565, Japanese Patent Application Laid-open No. H07-7435, Japanese Patent Application Laid-open No. 2002-314425, and Japanese Patent Application Laid-open No. 2006-42315.
A related-art signal modulation apparatus will now be specifically explained with reference to FIG. 7. FIG. 7 is a schematic explanatory diagram of the related-art signal modulation apparatus. The related-art signal modulation apparatus that includes an adder, an integrator, and a quantizer (comparator), and that modulates an input signal to a quantization signal will be explained.
On receiving the input signal, the integrator of the related-art signal modulation apparatus returns a previously self-generated and stored integrated signal to the adder (see (1) in FIG. 7), and the quantizer of the related-art signal modulation apparatus returns a previously self-generated quantization signal to the adder (see (2) in FIG. 7).
The adder of the related-art signal modulation apparatus generates an added signal, by subtracting the quantization signal returned by the quantizer from the input signal, and adding the integrated signal returned by the integrator thereto (see (3) in FIG. 7).
The integrator of the related-art signal modulation apparatus generates a new integrated signal by integrating the added signal generated by the adder, and stores the generated new integrated signal therein (see (4) in FIG. 7).
The quantizer of the related-art signal modulation apparatus generates a new quantization signal by quantizing the new integrated signal generated by the integrator (see (5) in FIG. 7).
In the related art, there is a problem that output noise generated by receiving a silent signal cannot be reduced.
In other words, with the audio equipment and the audio CODEC that the signal modulation apparatus is applied thereto, an improvement on the signal to noise ratio (SN ratio) of the signal modulation apparatus, and a reduction of noise when the silent signal is received, are required to improve the quality of actual sound.
However, in the related art that returns the integrated signal to the adder to improve the SN ratio of the signal modulation apparatus, when a silent signal is received after an audible signal is received as an input signal, the integrated signal generated when the audible signal is received, is added to the silent signal. Therefore, an audible quantization signal is generated, thereby occasionally generating noise as a consequence. In the related art, even if the silent signal is received, the integrated signal generated when the audible signal is received, is repeatedly returned to the adder without being attenuated. Accordingly, noise is sometimes generated, even if the silent signal is received.
For example, with the related art that improves quality of actual sound by adding a random signal made of short bits to the input signal, as disclosed in Japanese Patent Application Laid-open No. 2002-502565 and Japanese Patent Application Laid-open No. H07-7435, not only the installation is difficult, but noise is sometimes generated, even if the random signal receives the silent signal.